Mechanics

Newton's first and second laws of motion, in Latin, from the original 1687 edition of Principia Mathematica.

Mechanics (from the Greek term Μηχανική) is a branch of physics involving study of the movement of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment. This discipline, which has its roots in several ancient civilizations, is now subdivided into two main branches: classical mechanics and quantum mechanics.

During the early modern period, scientists such as Galileo, Johannes Kepler, and especially Isaac Newton, laid the foundations for what is now known as classical mechanics. The foundations of quantum mechanics were established during the first half of the twentieth century by Max Planck, Werner Heisenberg, Louis de Broglie, Albert Einstein, Niels Bohr, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Wolfgang Pauli and others. Quantum mechanics is now considered a foundational-level theory that encompasses and supersedes classical mechanics. However, classical mechanics is useful for calculations of macroscopic processes, while quantum mechanics helps explain and predict processes at the molecular, atomic, and subatomic levels.

Significance

Mechanics is the original discipline of physics and was formerly part of "natural philosophy," dealing with forces and motion in the macroscopic world as perceived by the human eye. This discipline has developed into a huge body of knowledge about important aspects of the natural world. Modern mechanics encompasses the movement of all matter in the universe under the four fundamental interactions (or forces): gravity, the strong and weak interactions, and the electromagnetic interaction.

Classical versus quantum mechanics

The major division of the discipline of mechanics is one that separates classical mechanics from quantum mechanics. Historically, classical mechanics came first, while quantum mechanics is a comparatively recent formulation. Classical mechanics originated with Isaac Newton's Laws of motion in Principia Mathematica, while quantum mechanics did not appear until 1900. Both are commonly held to constitute the most certain knowledge that exists about physical nature. Classical mechanics has especially often been viewed as a model for other so-called exact sciences. Essential in this respect is the relentless use of mathematics in theories, as well as the decisive role played by experiment in generating and testing them.

Quantum mechanics is of a wider scope, as it encompasses classical mechanics as a sub-discipline that is applicable under certain restricted circumstances. According to the correspondence principle, there is no contradiction or conflict between the two subjects, each simply pertains to specific situations. Quantum mechanics has superseded classical mechanics at the foundational level and is indispensable for the explanation and prediction of processes at molecular, atomic, and subatomic levels. However, for macroscopic processes, classical mechanics is able to solve problems that are unmanageably difficult in quantum mechanics and hence remains useful and well used.

Einsteinian versus Newtonian physics

Analogous to the quantum reformation of classical mechanics, Einstein's general and special theories of relativity have expanded the scope of mechanics beyond the mechanics of Newton and Galileo, and made fundamental corrections to them, that become significant and even dominant as speeds of material objects approach the speed of light, which cannot be exceeded.

Relativistic corrections are also needed for quantum mechanics, although relativity has not been fully integrated with it yet. This is one of the hurdles that has to be overcome in developing a Grand Unified Theory.

Types of mechanical bodies

The often-used term body needs to stand for a wide assortment of objects, including particles, projectiles, spacecraft, stars, parts of machinery, parts of solids, parts of fluids (gases and liquids), and so forth.

Other distinctions between the various sub-disciplines of mechanics, concern the nature of the bodies being described. Particles are bodies with little-known internal structure, treated as mathematical points in classical mechanics. Rigid bodies have size and shape, but retain a simplicity close to that of the particle, adding just a few so-called degrees of freedom, such as orientation in space.

Otherwise, bodies may be semi-rigid, that is, elastic, or non-rigid, that is, fluid. These subjects have both classical and quantum divisions of study.

For instance, the motion of a spacecraft, regarding its orbit and attitude (rotation), is described by the relativistic theory of classical mechanics. Analogous motions of an atomic nucleus are described by quantum mechanics.

Sub-disciplines of mechanics

The following two lists indicate various subjects that are studied under classical mechanics and quantum mechanics.

Classical mechanics

The following areas are included as part of the field of classical mechanics:

Newtonian mechanics, involves the original theory of motion (kinematics) and forces (dynamics)

Lagrangian mechanics, a theoretical formalism, based on the principle of conservation of energy

Hamiltonian mechanics, another theoretical formalism, based on the principle of the least action

Celestial mechanics, the motion of heavenly bodies, such as planets, comets, stars, and galaxies

Astrodynamics, for the navigation of spacecraft and similar objects

Solid mechanics, involving study of elasticity and the properties of (semi-)rigid bodies

Quantum statistical mechanics, dealing with large assemblies of particles

In addition to the above areas, there is the "theory of fields," which constitutes a separate discipline in physics, formally treated as distinct from mechanics, whether classical fields or quantum fields. But in actual practice, subjects belonging to mechanics and fields are closely interwoven. Thus, for instance, forces that act on particles are frequently derived from fields (electromagnetic or gravitational), and particles generate fields by acting as sources. In fact, in quantum mechanics, particles themselves are fields, as described theoretically by the wave function.

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